The question of 6 DOFs of Platform

Wind & Water Structural Analysis
1

Dear Jason,

I am a graduate student using FASTv8 and OPENFAST. I am studying 10MW jacket offshore wind turbine. And the jacket and tower are modeled in Subdyn, there is no platform for the jacket. The attachment are the ElastoDyn and SubDyn. I have two questions for you.

  1. I find that the opening and closing of the six degrees of freedom of the platform in the Elastondyn module will lead to a large difference in the first-order fundamental frequency. Therefore, what does the platform degree of freedom mean for a model like mine?
  2. Still about the six degrees of freedom of the platform of The Elastondyn module. Since there is no platform in my model, how does the six degrees of freedom of the Elastondyn module affect the calculation of FAST, and what is their internal mechanism? The platform with six degrees of freedom is confusing to me.

Looking forward to your reply.

Best regards
10MW_model.zip (8.57 KB)

2

Dear Dongzhe.Lu,

The ElastoDyn and SubDyn modules of FAST v8 and OpenFAST are coupled via structure-to-structure coupling where the motions in 6 DOFs (displacements/rotations, velocities, accelerations) are transferred from ElastoDyn to SubDyn and the reaction loads (forces, moments) are transferred from SubDyn to ElastoDyn. The 6-DOF motion is referred to as the platform DOFs in ElastoDyn and the interface joint in SubDyn.

In your case, you’ve located the platform in ElastoDyn and the interface joint in SubDyn at the top of the tower, meaning the full support structure (tower + jacket) are modelled in SubDyn, and the tower geometry is zeroed and the tower DOFs are disabled in ElastoDyn. That looks like the correct approach and all 6 platform DOFs should be enabled in ElastoDyn

More information is provided in the online SubDyn documentation: openfast.readthedocs.io/en/main … e-coupling.

Best regards,

3

Dear Jason,

Thank you very much for your reply, and I needed to use a specific wind speed time history, so I used WindType=2 (Uniform) for only wind calculation. WaveMod=0 (Stillwater) in the HydroDyn. The DT is 0.001, but the structural response had a large amplitude at about 200’s according to the settings of ElastoDyn to SubDyn. The attachment are the input wind speed time history. hh file and the structure response result.

I’m not sure what causes this problem, I would love to have your suggestions or ideas.

Best regards!
output.zip (2.3 MB)
wind1.3_600.zip (1023 Bytes)

4

Dear Dongzhe.Lu,

I’m not sure what is causing the thrust and tower-top oscillations at 200 s, etc. based only on the information you’ve provided. What aerodynamic options have you set? Do you see similar behavior for outputs within the support structure? What happens if you use steady wind inflow? What happens if you disable the drivetrain and blade DOFs in ElastoDyn?

Best regards,

5

Dear Jason,

Thank you very much for your reply.
When I changed the StallMod to STEADY in the Aerodyn module, I didn’t have a problem with large oscillations. And I found that the tower-top FX=thrust=tower-base FX , so I guess my tower is not under wind load.
The tower and jacket foundation are established in SubDyn module, so how should the wind load on the tower be simulated? How do I load the collected wind field data into the model above the MSL?

Best regards!
AeroDyn.zip (75 KB)

6

Dear Dongzhe.Lu,

Neither FAST v8 nor OpenFAST currently support direct wind loading on a tower modeled in SubDyn.

Please note that direct wind loading on the tower typically has a negligible effect on structural loading if the rotor is operational. Direct wind loading on a tower can be important when the rotor is parked or idling with blades feathered in high winds.

Best regards,

7

Dear Jason,

I need to do a simulation with idling rotor. I am modelling tower in SubDyn. As you said, ‘Direct wind loading on a tower can be important when the rotor is parked or idling with blades feathered in high winds’.
Is direct wind loading possible now in OpenFAST when tower is modelled in SubDyn? Or is there any other way?

Best regards,

8

Dear @Tom.Jose,

Unfortunately, not yet. That enhancement to add aerodynamic loading on multi-member towers with coupling between AeroDyn and SubDyn is near the top of our wish list of enhancements needed in OpenFAST, but NREL has not found a project that will fund that development yet.

Best regards,